9 If today any heathen Emperor should build an altar, which God forbid, to idols, and should compel Christians to come together there, in order to be among those who were sacrificing, so that the smoke and ashes from the altar, the sparks from the sacrilege, the smoke from the burning might choke the breath and throats of the faithful; and should give judgment in that court where members were compelled to vote after swearing at the altar of an idol (for they explain that an altar is so placed for this purpose, that every assembly should deliberate under its sanction, as they suppose, though the Senate is now made up with a majority of Christians), a Christian who was compelled with a choice such as this to come to the Senate, would consider it to be persecution, which often happens, for they are compelled to come together even by violence.
Are these Christians, when you are Emperor, compelled to swear at a heathen altar? What is an oath, but a confession of the divine power of Him Whom you invoke as watcher over your good faith? When you are Emperor, this is sought and demanded, that you should command an altar to be built, and the cost of profane sacrifices to be granted.
Source: Letters (New Advent)